Anaerobic fermentations of hydrogen and carbon monoxide involve the contact of a gaseous substrate-containing feed with an aqueous fermentation menstruum containing microorganisms capable of generating oxygenated organic compounds such as ethanol, acetic acid, propanol and n-butanol. The bioconversion of carbon monoxide results in the production of oxygenated organic compound and carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion or, as used herein, the hydrogen conversion.
Sulfur is a key nutritional need of anaerobic microorganisms used in these fermentations to produce oxygenated organic compound. Organic sulfur sources, such as cysteine, have been used to provide the nutritional sulfur. These organic sulfur sources are expensive, and alternative sources of sulfur to meet this nutritional need have been sought. Less expensive sources of sulfur include, but are not limited to, hydrogen sulfide, and sulfite, bisulfite, thiosulfate and metabisulfite anions. However, typical aqueous menstrua for the bioconversion of carbon monoxide and of hydrogen and carbon dioxide are acidic. Consequently the equilibrium for hydrogen sulfide, which provides the sulfhydryl anion that is believed to be used by the microorganisms, strongly favors gaseous hydrogen sulfide as opposed to the sulfhydryl anion, and gaseous hydrogen sulfide rapidly exits the aqueous menstruum.
To maintain available sulfur nutrient in view of the evolution of hydrogen sulfide from the aqueous menstruum, the sulfur nutrient is typically added in an amount much greater than that metabolically required. This overdosing increases operating costs due to the amount of sulfur nutrient required to be supplied. In addition, the off-gas contains hydrogen sulfide in problematic concentrations. Hence, accommodations may be required to remove or reduce the concentration of hydrogen sulfide in the off-gas to enable the use or disposal of the off-gas and to attenuate corrosive properties of the off-gas.
Accordingly, processes for the anaerobic conversion of carbon monoxide and of hydrogen and carbon dioxide to oxygenated organic compounds are sought that can reduce the amount of sulfur nutrient required to be supplied to the aqueous menstruum and attenuate the concentration of hydrogen sulfide in the off-gas.